1. Field of the Invention
The present invention relates to a chromatography system, a signal processing apparatus, a chromatography data processing apparatus, and a program. More particularly, the present invention relates to technology of reducing variations of a baseline.
2. Description of the Related Art
One of general detectors used for liquid chromatography includes a multi-channel detection device employing a photodiode array or a solid-state imaging device, which is hereinafter referred to as a PDA detector. The PDA detector is configured to perform detection with use of specimen's characteristics of absorbing light having specific wavelengths. For example, the PDA detector may measure the intensity of light incident to a specimen and the intensity of light emitted from the specimen and then calculate an absorbance from a difference between those measured intensities or calculate a concentration of the specimen from such a calculated absorbance.
In a PDA detector, components to be analyzed is flowed and measured through a flow cell. One of analysis methods used for liquid chromatography includes a gradient elution method, which uses a plurality of eluants having different characteristics such as pH or concentration and varies the composition of the eluants in a continuous manner as the analysis progresses. Thus, the type and composition of a liquid within a flow cell is varied as the analysis progresses in a gradient elution method. However, any liquid has an inherent index of refraction. Therefore, the index of the interior of the flow cell is varied. This variation causes light to be emitted from the flow cell to a different degree of refraction, resulting in varied intensities of light that reaches the detector. Since the intensity of light that reaches the detector is thus varied, a phenomenon that the absorbance is seemingly varied is encountered. With analysis using a gradient elution method, this phenomenon causes the shape of the baseline to be changed into a curved line due to the influence of the index of refraction even if an eluant does not absorb light. Furthermore, in a detector for measuring an absorbance, such as a PDA detector, such changes adversely affect the accuracy and precision of qualitative analysis or quantitative analysis.
In order to suppress such variations of a baseline which results from a gradient elution method, for example, a tapered flow cell as shown in FIG. 1(b) has been used instead of a general flow cell (straight type flow cell) in which a passage is cylindrical as shown in FIG. 1(a). Either flow cell has a body 1 made of stainless, for example, an incident window plate 2 made of silica glass, for example, and an emission window plate 3 made of silica glass, for example. A passage 4 extends through the body 1. The incident window plate 2 is disposed on one end of the passage 4 (on the left end of the passage 4 in FIGS. 1(a) and 1(b)). The emission window plate 3 is disposed on the other end of the passage 4 (on the right end of the passage 4 in FIGS. 1(a) and 1(b)). As is apparent from FIGS. 1(a) and 1(b), the passage 4 of the straight type flow cell is in the form of a cylinder having an inside diameter that is uniform over the length of the passage 4 (see FIG. 1(a)), and the passage 4 (see FIG. 1(b)) of the tapered flow cell is in the form of a truncated cone having an inner shape gradually widened along the travelling direction of the light. For example, JP-B H08-3483 discloses such a tapered flow cell.
For example, upon use of a general flow cell (straight type flow cell) in which a passage is cylindrical as shown in FIG. 1(a), if incident light is focused at the vicinity of an inlet of the flow cell, it impinges on a wall downstream of the passage, thereby causing losses. Accordingly, the intensity of light that reaches the detector is lowered.
In contrast, with a tapered flow cell of JP-B H08-3483 as shown in FIG. 1(b), for example, an angle of the incident light coincides with an angle of a wall of an inner circumferential surface of the passage 4 in the flow cell. Thus, the incident light is designed to be unlikely to impinge on the wall of the passage 4. Accordingly, variations in intensity of emitted light that are caused by variations in index of refraction can be suppressed.
Meanwhile, a deuterium lamp, a tungsten lamp, or the like is generally used in a PDA detector. Those lamps have characteristics that the intensity of emitting light varies depending upon variations of environmental temperatures. The variations of environmental temperatures reflect the measurement results as a drift of the baseline. The lamp is a heat source as well as a light source. If the lamp continuously emits light, the surrounding environmental temperature increases due to heat produced by the lamp. Particularly, with liquid chromatography, measurement is performed continuously for several minutes to several tens of minutes. Therefore, variations of the environmental temperatures greatly affect the drift of the baseline. In order to solve such a problem, there has been proposed to provide an air-cooling mechanism in a light source room that accommodates a light source as disclosed in JP-B 4419637. This cooling mechanism controls the temperature of the light source room to be constant.
As described above, various methods have heretofore been proposed to improve the drift of the baseline. However, those methods do not demonstrate satisfactory effects. For example, with the tapered flow cell disclosed in JP-B H08-3483, light incident on the flow cell is refracted at an interface between air and quartz and an interface between quartz and an eluant (liquid) because the indexes of refraction differ between air and quartz and between quartz and the eluant. In a gradient elution method, the type of the eluant is changed as the analysis progresses. As a result, the index of refraction between the cell window plate and the eluant changes. Accordingly, light that would not come out because of impingement on a wall of the flow cell (see FIG. 1(a)) is allowed to come out. Thus, the intensity of light that reaches a detector increases.
As described above, in a tapered flow cell, an angle of incident light coincides with an angle of a wall of the flow cell such that incident light is unlikely to impinge on the wall. However, the incident light cannot completely be controlled when the index of refraction varies to a large degree as shown in FIG. 1(c).
The method of stabilizing the operating temperature of a lamp as disclosed in JP-B 4419637 may require an increased number of parts, resulting in an increased cost of production. Furthermore, with such a method, a function of adjusting the temperature may not properly work when the room temperature changes due to air-conditioning or the like.